1. Field of the Invention
The present invention relates to a coating composition, more particularly to a coating composition which provides an exterior overcoating having an excellent weathering resistance, solvent resistance, and dirt resistance, and yet having a good appearance, undercoat adhesiveness, and recoating adhesiveness. The present invention also relates to a coating composition by which a coating having a deep tone can be formed by ensuring that the upper layer of the pigment dispersed system is a clear layer. Particularly, it relates to a coating composition which can impart a characteristic of the resin coating in a small amount to the resin of the clear layer which becomes the upper layer of the pigment dispersed system. These coating compositions can be used as final-coat paints for automobiles, paints for exteriors of buildings, and paints for metal substrates, etc.
2. Description of the Related Art
In the final-coating of automobiles, etc., or exterior coating of construction materials, etc., coatings having an excellent appearance and an excellent weathering resistance, solvent resistance, dirt resistance, etc., are required, but although top-coats such as acrylic resin paints, alkyl resin paints, polyester resin paints, conventionally used for general purpose work have an excellent coating appearance, they have a poor weathering resistance, solvent resistance, and dirt resistance, and an improvement of these properties is desired. Accordingly, various proposals have been made for solving such problems; for example, it has been proposed to coat a fluorine type resin on the top-coat, or to use a fluorine type resin as the resin for the top-coat. Japanese Unexamined Patent Publication (Kokai) No. 57-34107 (Japanese Patent Publication (Kokoku) No. 60-21686) discloses a fluorine-containing copolymer curable at a normal temperature. According to this publication, by using a copolymer containing a specific amount of fluoroolefin, cyclohexylvinyl ether, alkylvinyl ether, and hydroxyalkylvinyl ether as essential constituents, not only is the curability improved to a great extent, to enable curing within a short time by use of a known curing agent such as melamine, etc., but also a weathering-resistant coating having an enriched gloss and excellent solvent resistance, which is curable at a normal temperature by the use of isocyanate, can be obtained.
Also, Japanese Unexamined Patent Publication (Kokai) No. 59-197471 described a thermosetting resin composition for paint comprising a fluorine-containing copolymer. This publication disclosed that a solid color paint, metallic paint, clear paint, etc., having an excellent water resistance, solvent resistance, weathering resistance, etc., can be obtained by using a thermosetting resin composition for paint comprising a mixture of copolymers of a fluorine-containing copolymer containing a hydroxyl group and a copolymer comprising an alkyl ester of acrylic acid or methacrylic acid with other acrylic monomers, and a curing agent containing a functional group reactive with the hydroxyl group.
Further, Japanese Unexamined Patent Publication (Kokai) No. 61-46283 disclosed a paint composition containing a fluorine-containing copolymer. According to this publication, a coating having an excellent weathering resistance over a long term and an excellent appearance with a high gloss and good sharpness can be obtained by applying, as the final coating, a top clear paint comprising a polyol component comprising a fluorine-containing copolymer comprising fluoroolefin, cyclohexylvinyl ether, alkylvinyl ether, hydroxyalkylvinyl ether, and another copolymer polymerized at specific proportions, and an acrylic copolymer, and a polyhydric isocyanate compound or an aminoplast compound, on the base paint coating.
However, these coating compositions of the prior art contain a copolymer resin such as a fluorine-containing resin, and when a powder paint or water-dispersion paint is used, require baking at a high temperature have a poor coating appearance, and further, the solvent soluble type is very expensive and has a drawback in that the adhesiveness to the undercoat or recoating adhesiveness are poor.
Further, there was a problem in that the pigment-dispersed type top-coat of the prior art has unsatisfactory coating characteristics when the coating appearance is preferentially taken into account, and conversely, has an inferior appearance when the coating performance is preferentially taken into account. Accordingly, it has been very difficult in the prior art to satisfy both of these two antagonistic requirements.
As described above, attempts have been made in the prior art to coat a fluorine type resin on the exterior coating or formulate the resin in the top-coat paint, to improve the properties of an exterior top-coat, such as weathering resistance, solvent resistance, and dirt resistance, but problems still remain in that the appearance is bad, as mentioned above, and the adhesiveness to the undercoat or recoating adhesiveness are also bad. Conversely, if an attempt is made to improve the appearance of the coating, a problem arises in that coating properties such as weathering resistance, solvent resistance, and dirt resistance, etc., become unsatisfactory.